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Chapter 5: Landfill Gas Control Measures

Historical Document

This document is provided by the Agency for Toxic Substances and Disease Registry (ATSDR)
ONLY as an historical reference for the public health community. It is no longer being maintained and the data
it contains may no longer be current and/or accurate.

This chapter presents an overview of common landfill
gas control technologies. These technologies include means to collect
gases, control and treat gases, and use gases to benefit the community
(e.g., to generate electricity or heat buildings). A landfill might
need gas control measures for several reasons, including government
regulations, odor problems, or uncontrolled releases of gases that
could pose safety and health concerns. As an environmental health
professional, you are not expected to be able to design and implement
a landfill gas control plan. However, you should have a basic understanding
of the control options that are available to help prevent or control
exposures to landfill gas.

Why would control measures be implemented at
a landfill?

Many landfills install gas control measures because of regulatory
requirements. The federal government has developed laws and regulations
that govern the operation and maintenance of landfills. These regulations
have been developed to reduce health and environmental impacts from
landfill gas emissions through the reduction of ozone precursors
(volatile organic compounds and nitrogen oxides), methane, NMOCs,
and odorous compounds. States may also have statespecific landfill
regulations, which must be as strict or more strict than the federal
regulations. The boxes on the next page review some of the applicable
regulations.

As described in Chapter Three, odor complaints
or potential safety and health concerns may also prompt landfill
gas collection. Sulfide emissions are a common source of landfill
odor complaints. At older landfills or at smaller landfills exempt
from federal and state regulations, uncontrolled releases of landfill
gases can pose potential safety and health concerns (e.g., explosion
hazards). In such cases, the landfill might implement landfill gas
control measures, even if they are not required by federal or state
regulations. Some landfills have also implemented voluntary gas
collection and control or treatment systems to recover landfill
gas for energy production.

What are the components of a landfill gas control
plan?

The goal of a landfill gas control plan is to prevent people
from being exposed to landfill gas emissions. This goal can be achieved
by either collecting and treating landfill gas at the landfill or
by preventing landfill gas from entering buildings and homes in
the community. Technologies used to control landfill gas at the
landfill or in the community can be applied separately or in combination.
Note that the NSPS/ EG requires a gas collection and control system
design plan for landfills that meet the criteria presented on the
next page. The NSPS rule specifies the type of information that
must be included and the criteria the collection and control systems
must meet.

Since October 1979, federal regulations promulgated under
Subtitle D of RCRA— which regulates the siting, design,
construction, operation, monitoring, and closure of MSW
landfills—have required controls on migration of methane
in landfill gas. These regulations do not address other
components of landfill gas. In 1991, EPA issued standards
for landfill design and performance that apply to MSW landfills
active on or after October 9, 1993. The standards require
methane monitoring and establish performance standards for
methane migration control. Monitoring requirements must
be met at landfills not only during their operation, but
also for a period of 30 years after closure.

Landfills affected by RCRA Subtitle D are required to
control gas by establishing a program to periodically check
for methane emissions and prevent off-site migration. Landfill
owners and operators must ensure that the concentration
of methane gas does not exceed:

25% of the EL for methane in the facilities' structures
(1.25% by volume)

The LEL for methane at the facility boundary (5%
by volume)

Permitted limits on methane levels reflect the fact that
methane is explosive within the range of 5% to 15% concentration
in air. If methane emissions exceed the permitted limits,
corrective action (i. e., installation of a landfill gas
collection system) must be taken. The Subtitle D RCRA regulations
for MSW landfills can be found in 40 CFR Part 258, which
can be viewed through EPA's Office of Solid Waste Web page
at

Federal Requirements Under the Clean Air Act
(CAA) Regulations (NSPS/EG)

Under NSPS/EG of the CAA, EPA requires affected landfills
to collect and control landfill gas. The NSPS/EG target
reductions in the emissions of landfill gas due to odor,
possible health effects, and safety concerns. The rules
use NMOCs (which contribute to local smog formation) as
a surrogate for total landfill gas to determine if control
is required. Landfills meeting certain design capacity and
emissions criteria are required to collect landfill gas
and either flare it or use it for energy. Landfills that
meet both of the following criteria must collect and control
landfill gas emissions.

Capacity: design capacity greater than or equal
to 2.5 Mg and 2.5 million cubic meters.

Emissions: annual NMOC emission rate greater than
or equal to 50 Mg.

The basic requirements are the same for both existing
and new landfills. Existing landfills are defined as landfills
that received waste after November 8, 1987, and began construction
before May 30, 1991. These are regulated through the EG.
New landfills are defined as landfills that began construction,
reconstruction, or modification on or after May 30, 1991.
These are subject to the NSPS. The CAA regulations (NSPS/EG)
for MSW landfills can be found in 40 CFR Part 60, Subparts
Cc and WWW, available on the Internet at
http://www.access.gpo.gov/nara/cfr/waisidx_00/40cfr60_00.html.
State plans and a federal plan to implement the EG for existing
landfills can be found in 40 CFR Part 62.You can also view
all Federal Register notices and summary information at
http://www.epa.gov/ttn/atw/landfill/landflpg.html.

How is landfill gas collected?

Landfill gas can be collected by either a passive or an active
collection system. A typical collection system, either passive or
active, is composed of a series of gas collection wells placed throughout
the landfill. The number and spacing of the wells depend on landfill-specific
characteristics, such as waste volume, density, depth, and area.
As gas is generated in the landfill, the collection wells offer
preferred pathways for gas migration, as discussed in
Chapter Two. Most collection systems are
designed with a degree of redundancy to ensure continued operation
and protect against system failure. Redundancy in a system may include
extra gas collection wells in case one well fails. The system-specific
components for passive and active gas collection systems are discussed
below.

Passive Gas Collection Systems.
Passive gas collection systems (Figure 5-1)
use existing variations in landfill pressure and gas concentrations
to vent landfill gas into the atmosphere or a control system.
Passive collection systems can be installed during active operation
of a landfill or after closure. Passive systems use collection
wells, also referred to as extraction wells, to collect landfill
gas. The collection wells are typically constructed of perforated
or slotted plastic and are installed vertically throughout the
landfill to depths ranging from 50% to 90% of the waste thickness.
If groundwater is encountered within the waste, wells end at
the groundwater table. Vertical wells are typically installed
after the landfill, or a portion of a landfill, has been closed.
A passive collection system may also include horizontal wells
located below the ground surface to serve as conduits for gas
movement within the landfill. Horizontal wells may be appropriate
for landfills that need to recover gas promptly (e.g, landfills
with subsurface gas migration problems), for deep landfills,
or for active landfills. Sometimes, the collection wells vent
directly to the atmosphere. Often, the collection wells convey
the gas to treatment or control systems (e.g., flares).

The efficiency of a passive collection system partly depends
on how well the gas is contained within the landfill. Gas containment
can be controlled and altered by the landfill collection system
design. Gas can be contained by using liners on the top, sides,
and bottom of the landfill. An impermeable liner (e. g., clay
or geosynthetic membranes) will trap landfill gas and can be
used to create preferred gas migration pathways. For example,
installing an impermeable barrier at the top of a landfill will
limit uncontrolled venting to the atmosphere by causing the
gas to vent through collection wells rather than the cover.

The efficiency of a passive collection system also depends on
environmental conditions, which may or may not be controlled
by the system design. When the pressure in the landfill is inadequate
to push the gas to the venting device or control device, passive
systems fail to remove landfill gas effectively. High barometric
pressure, as discussed in Chapter Two,
sometimes results in outside air entering the landfill through
passive vents that are not routing gas to control devices. For
these reasons, passive collection systems are not considered
reliable enough for use in areas with a high risk of gas migration,
especially where methane can collect to explosive levels in
buildings and confined spaces.

It is fairly common for landfills to flare gas due to odor concerns,
for example, even if not the landfill is not subject to regulatory
requirements. Passive gas collection systems may be used to
comply with the NSPS/ EG only at landfills where cells are lined
in accordance with Subtitle D of RCRA to prevent gas migration.

Figure 5-1: Passive
Gas Collection System

Active Gas Collection. Well-designed
active collection systems (Figure 5-2) are
considered the most effective means of landfill gas collection
(EPA 1991). Active gas collection systems include vertical and
horizontal gas collection wells similar to passive collection
systems. Unlike the gas collection wells in a passive system,
however, wells in the active system should have valves to regulate
gas flow and to serve as a sampling port. Sampling allows the
system operator to measure gas generation, composition, and
pressure.

Active gas collection systems include vacuums or pumps to move
gas out of the landfill and piping that connects the collection
wells to the vacuum. Vacuums or pumps pull gas from the landfill
by creating low pressure within the gas collection wells. The
low pressure in the wells creates a preferred migration pathway
for the landfill gas. The size, type, and number of vacuums
required in an active system to pull the gas from the landfill
depend on the amount of gas being produced. With information
about landfill gas generation, composition, and pressure, a
landfill operator can assess gas production and distribution
changes and modify the pumping system and collection well valves
to most efficiently run an active gas collection system. The
system design should account for future gas management needs,
such as those associated with landfill expansion. The box on
the next page describes components of an effective active gas
collection system.

Figure 5-2: Active Gas Collection System

How Is an Effective Active Gas System Designed?

An effective active gas collection system incorporates
the following design elements (EPA 1991):

Collection wells placed to capture gas from all
areas of the landfill. The number and spacing between
each extraction well depends on the waste type, depth,
and compaction; the pressure gradients created by the
vacuums; and the moisture content of the gas.

The ability to monitor and adjust flow from individual
extraction wells. Inclusion of a valve, pressure gauge,
condenser, and sampling port at each collection well
allows a landfill operator to monitor and adjust pressure
and to measure gas generation and content

What methods are available to treat landfill
gas after collection?

Some passive gas collection systems simply vent landfill gas
to the atmosphere without any treatment before release. This may
be appropriate if only a small quantity of gas is produced and no
people live or work nearby. More commonly, however, the collected
landfill gas is controlled and treated to reduce potential safety
and health hazards. (A landfill may be required to do so by law,
such as the NSPS/ EG, as described in Chapter
Four.) Common methods to treat landfill gas include combustion
and noncombustion technologies, as well as odor control technologies.

Combustion. Combustion is the most
common technique for controlling and treating landfill gas.
Combustion technologies such as flares, incinerators, boilers,
gas turbines, and internal combustion engines thermally destroy
the compounds in landfill gas. Over 98% destruction of organic
compounds is typically achieved. Methane is converted to carbon
dioxide, resulting in a large greenhouse gas impact reduction.
Combustion or flaring is most efficient when the landfill gas
contains at least 20% methane by volume. At this methane concentration,
the landfill gas will readily form a combustible mixture with
ambient air, so that only an ignition source is needed for operation.
At landfills with less than 20% methane by volume, supplemental
fuel (e. g., natural gas) is required to operate flares, greatly
increasing operating costs. When combustion is used, two different
types of flares can be chosen: open or enclosed flares.

Open flame flares (e. g., candle or pipe flares),
the simplest flaring technology, consist of a pipe through
which the gas is pumped, a pilot light to spark the gas,
and a means to regulate the gas flow. The simplicity of
the design and operation of an open flame flare is an advantage
of this technology. Disadvantages include inefficient combustion,
aesthetic complaints, and monitoring difficulties. Sometimes,
open flame flares are partially covered to hide the flame
from view and improve monitoring accuracy.

Enclosed flame flares are more complex and expensive
than open flame flares. Nevertheless, most flares designed
today are enclosed, because this design eliminates some
of the disadvantages associated with open flame flares.
Enclosed flame flares consist of multiple burners enclosed
within fire- resistant walls that extend above the flame.
Unlike open flame flares, the amount of gas and air entering
an enclosed flame flare can be controlled, making combustion
more reliable and more efficient.

Other enclosed combustion technologies such as
boilers, process heaters, gas turbines, and internal combustion
engines can be used not only to efficiently destroy organic
compounds in landfill gas, but also to generate useful energy
or electricity, as described later in this chapter.

Some public concerns have been raised about whether the combustion
of landfill gas may create toxic chemicals. Combustion can create
acid gases such as SO2 and NO X . The generation of dioxins
has also been questioned. EPA investigated the issue of dioxin formation
and concluded that the existing data from several landfills did
not provide evidence showing significant dioxin formation during
landfill gas combustion. Because of the potential imminent health
threat from other components of landfill gas, landfill gas destruction
in a properly designed and operated control device, such as a flare
or energy recovery unit, is preferable to uncontrolled release of
landfill gas. Scientists continue to review new information on by-
product emissions from landfill gas control devices as it becomes
available.

Noncombustion. Noncombustion technologies
were developed in the 1990s as an alternative to combustion,
which produces compounds that contribute to smog, including
nitrogen oxides, sulfur oxides, carbon monoxide, and particulate
matter. Noncombustion technologies fall into two groups: energy
recovery technologies and gas-to-product conversion technologies.
Regardless of which noncombustion technology is used, the landfill
gas must first undergo pretreatment to remove impurities such
as water, NMOCs, and carbon dioxide. Numerous pretreatment methods
are available to address the impurities of concern for a specific
landfill. After pretreatment, the purified landfill gas is treated
by noncombustion technology options.

Energy recovery technologies use landfill gas
to produce energy directly. Currently, the phosphoric acid
fuel cell (PAFC) is the only commercially available noncombustion
energy recovery technology. Other types of fuel cells (molten
carbonate, solid oxide, and solid polymer) are still under
development. The PAFC system consists of landfill gas collection
and pretreatment, a fuel cell processing system, fuel cell
stacks, and a power conditioning system. Several chemical
reactions occur within this system to create water, electricity,
heat, and waste gases. The waste gases are destroyed in
a flare.

Gas-to-product conversion technologies focus
on converting landfill gas into commercial products, such
as compressed natural gas, methanol, purified carbon dioxide
and methane, or liquefied natural gas. The processes used
to produce each of these products varies, but each includes
landfill gas collection, pretreatment, and chemical reactions
and/ or purification techniques. Some of the processes use
flares to destroy gaseous wastes.

Odor Control Technologies.Odor control
technologies prevent odor-causing gases from leaving the landfill.
Installing a landfill cover will prevent odors from newly deposited
waste or from gases produced during bacterial decomposition.
Covering a landfill daily with soil can help reduce odors from
newly deposited wastes. More extensive covers are installed
at landfill closure to prevent moisture from infiltrating the
refuse and encouraging bacterial growth and decomposition. Vegetative
growth on the landfill cover also reduces odors. Flaring is
another technique that can eliminate landfill gas odors by thermally
destroying the odor-causing gases. Venting landfill gas through
a filter is another technology used to reduce odors. Landfill
gas is collected and vented through a filter of bacterial slime.
As long as oxygen is present, bacteria will decompose landfill
gas under aerobic conditions, producing carbon dioxide and water.
See the example below of odor controls used at a landfill in
California.

Odor Control at the Calabasas Landfill

The Calabasas Landfill, serving 1.4 million people in
the Los Angeles area, received approximately 17 million
tons of waste from its inception in 1961 through December
1995, when the County of Los Angeles passed an ordinance
limiting its use.

Beginning in the mid-1980s, an active landfill gas collection
system was installed in phases. The system consists of a
network of vertical wells and horizontal trenches placed
throughout the refuse fill. A vacuum is applied to the system
of wells and trenches to draw the gas into the collection
system. The collected gas is routed to a flare station and
combusted in flares.

The gas collection system, along with rejection of odorous
loads and application of daily cover, is a primary means
of controlling odor at the landfill. As a result of these
measures, the facility received only one odor complaint
during 1995 (NPS 1997).

What methods are available to control landfill
gas if it reaches nearby structures?

Under certain conditions, landfill gas migrating underground
from the landfill to the surrounding community could present safety
and health hazards, such as explosion or asphyxiation hazards. (see
Chapter Three for a more detailed discussion
of these hazards.) Once landfill gas reaches a building or home,
it can enter the structure through a number of available pathways
(as shown in Chapter Three,
Figure 3- 1).

To prevent landfill gas from entering buildings, controlling
the gas at the source (the landfill) is the preferred approach.
However, several simple community-based or structure-based controls
are available to reduce the gas entry pathways and limit indoor
migration of gas. If a landfill gas problem is anticipated before
construction, control strategies can be incorporated into the building
design. If not, alterations to the finished structure might be needed.
The two basic approaches to preventing gases from entering a structure
include controlling the gas pressure and eliminating available entry
pathways or leaks. Regardless of the methods used to prevent or
reduce landfill gas entry, continuous methane monitors with appropriate
alarms should be strategically placed in buildings where accumulation
of explosive levels of landfills gases is possible. The methane
monitors and engineering controls should have a frequent safety
check and maintenance program to ensure proper function. The box
below details the limitations of different landfill gas control
options.

Gas Pressure Controls.If gas pressure
is lower inside a building or structure than it is in the surrounding
soils, gas will flow into the building or structure. Controlling
gas pressure, therefore, can prevent gas migration indoors.
Some techniques to control gas pressure include passive or active
venting to reduce gas concentrations under the house, venting
around the perimeter of the house, and crawl- space venting.
Some of these techniques, however, may require pumps with maintenance
and energy requirements.

Leakage Area Controls.Another strategy
to prevent gas from entering a building or structure is to reduce
or eliminate entry pathways. Gas can leak into a building or
structure through cracks, gaps, drainage pipes, fireplace air
vents, and air conditioning or duct work. Improving plumbing
and caulking in a basement to reduce cracks and gaps will reduce
entry pathways. These options, however, may only partially address
indoor gas migration. Another control option is to install a
low-permeability liner around the basement or underground portion
of the building.

Passive venting is effective only with low underground
gas concentrations.

Active venting may require maintenance.

Leakage Area Controls

Plumbing corrections may only partially remedy the
problem.

Use of sealing, caulking, and liners has had limited
success gas migration. Another control option is to
install a low-permeability liner around the basement
or underground portion of the building.

Are there any beneficial uses for collected landfill
gas?

Are there any beneficial uses for collected
landfill gas?

Landfill gas is the single largest source of man-made methane
emissions in the United States, contributing to almost 40% of methane
emissions each year (EPA 1996). Consequently, a growing trend at
landfills across the country is to use recovered methane gas from
landfills as an energy source. Collecting landfill gas for energy
use greatly reduces the risk of explosions, provides financial benefits
for the community, conserves other energy resources, and potentially
reduces the risk of global climate change.

Currently in the United States, approximately 325 landfill gas
energy recovery projects prevent emissions of over 150 billion cubic
feet of methane per year (or more than 300 billion cubic feet of
landfill gas). Approximately 220 of these projects generate electricity,
producing a total of more than 900 megawatts per year. Another 68
projects are under construction in 2001, and more than 150 additional
projects are in the planning stages. Previous studies by EPA and
the Electric Power Research Institute estimate that up to 750 of
the landfills in the United States could profitably recover and
use their methane emissions (DOE n.d.a.).

What landfills can be used for gas recovery
and how is energy generated from landfill gas?

The feasibility of installing a landfill gas recovery system
depends on factors such as landfill gas generation rates, the availability
of users, and the potential environmental impacts. Many different
landfill types with varying gas production rates and composition
can support energy recovery projects. There are, however, several
guidelines to consider when assessing the feasibility of generating
energy from landfill gas. The box on the following
page lists some of these guidelines.

If feasible, energy recovery can be implemented by use of combustion-
or noncombustion- based technologies. Combustion-based technologies
that recover energy include boilers, process heaters, gas turbines,
and internal combustion engines. For example, landfill gas can be
piped to a nearby industry, commercial business, school or government
building where it is combusted in a boiler to provide steam for
an industrial process or heat for a building. It may be combusted
in an industrial process heater to provide heat for a chemical reaction.
Turbines and internal combustion engines can combust landfill gas
to generate electricity. The electricity can be used to meet power
needs at the landfill or a nearby facility, or the electricity may
be sold to the power grid.

The choice of which type of combustion device to use (e. g.,
boiler, gas turbine, internal combustion engine) depends on what
users are located near the landfill, site-specific technical and
economic considerations, and sometimes environmental impacts. For
example, internal combustion engines are often less costly than
gas turbines for smaller landfills. However, these engines may emit
more NOx , which contributes to ozone formation. If the landfill
is in a nonattainment area for ozone, then NOx emissions may be
a barrier to using an internal combustion engine.

Information on typical emissions from various combustion devices
can be found in EPA's compilation of air pollutant emission factors
(AP- 42). Information on these technologies can also be found in
the background document for the NSPS/EG (EPA 1991) and on the Landfill
Methane Outreach Program (LMOP) Web site at
http://www.epa.gov/lmop.

Landfill gas recovery systems cite the following factors
as guidelines important for economically feasible landfill
gas recovery projects. However, new technologies are becoming
available that have allowed successful projects at smaller
landfills. For example, smaller landfills can generate enough
gas to heat an on- site greenhouse or to use a microturbine
to generate a small amount of electricity. Various federal
and state incentives (e. g., grants, loans, tax credits,
renewable energy purchase requirements) can also enhance
the economic feasibility of landfill gas recovery projects.

The amount of waste in place at a landfill is greater
than approximately 1 million tons.

The waste is greater than 35 feet deep and is stable
enough for well installation.

The landfill area is greater than 35 acres.

The landfill is composed of refuse that can generate
large quantities of landfill gas composed of 35% or
more of methane. An industry guideline states that gas
recovery is economically viable at landfills with gas
generation rates of 1 million cubic feet per day (EPA
1996).

If a landfill is still open, active landfill operation
will continue for several more years.

If a landfill is already closed, a short time (no
more than a few years) has elapsed since closure.

The climate is conducive to gas production (very
cold or very dry climates can inhibit gas production).

The energy user is located nearby or in an area
accessible to the landfill.

Noncombustion energy recovery systems are also available, but
are not used as widely. Fuel cells are a promising new technology
for producing energy from landfill gas that does not involve combustion.
This technology has been demonstrated and in the future may become
more economically competitive with other options. One option that
does not involve combustion of landfill gas at or near the landfill
is purifying the landfill gas to remove constituents other than
methane, producing a high British thermal unit (Btu) gas that can
be sold as pipeline quality natural gas. While the high Btu gas
is eventually combusted, it would not contribute to any emissions
near the landfill. Another option is using compressed landfill gas
as a vehicle fuel.

Both combustion and noncombustion energy recovery systems have
three basic components: (1) a gas collection system; (2) a gas processing,
treatment, and conversion system; and (3) a means to transport the
gas or final product to the user (Figure 5-3).
Gas is collected from the landfill by the use of active vents. It
is then transported to a central point for processing. Processing
requirements vary, depending on the gas composition and the intended
use, but typically include a series of chemical reactions or filters
to remove impurities. For direct use of landfill gas in boilers,
minimal treatment is required. For landfill gas injection into a
natural gas pipeline, extensive treatment is necessary to remove
carbon dioxide. At a minimum, the gas is filtered to remove any
particles and water that may be suspended in the gas stream.

Some examples of successful landfill gas to energy projects are
presented in the box below. For more information
about landfill gas-to-energy projects, visit the EPA's Landfill
Methane Outreach Program (LMOP) Web site at
http://www.epa.gov/lmop.

Figure 5-3: Landfill
Gas Recovery System

Reusing Landfill Gas: Success Stories

Below are some examples of how gas collected from landfills
is being reused for power.

In Raleigh, North Carolina, Ajinomoto Pharmaceutical
Company has used landfill gas as fuel in boilers at
its facility since 1989. The steam produced by the boilers
is used to heat the facility and warm pharmaceutical
cultures. This project has prevented pollution equivalent
to removing more than 23,000 cars from the road.

In Pittsburgh, Pennsylvania, Lucent Technologies
saves $100,000 a year on fuel bills by using landfill
gas to generate steam for space heating and hot water.

The City of Riverview, Michigan, works with the
local utility, Detroit Energy, to recover landfill gas
and create electricity with two gas turbines. The project
generates enough power to meet the energy needs of more
than 3,700 homes.

The Los Angeles County Sanitation District in California
has succeeded in turning landfill gas into a clean alternative
vehicle fuel. Landfill gas is compressed to produce
enough fuel per day to run an 11-vehicle fleet, ranging
from passenger vans to large on- road tractors.

Pattonville High School in Maryland Heights, Missouri,
is located within 1 mile of a municipal solid waste
landfill. The landfill supplies methane gas to heat
the 4, 000- square- foot high school, saving the Pattonville
School District thousands of dollars in annual heating
costs. Pattonville High School was the first high school
to use landfill gas as its source of heat (CNN 1997)

SWANA. 1997. Solid Waste Association of North America. Landfill
Gas Operation and Maintenance Manual of Operation. SR-430-23070.
Available by searching the Department of Energy Information
Bridge at the Web site
http://www.osti.gov.